1. Field of the Invention
The present invention relates to fuses and, more particularly, to a fusible element of a high voltage current limiting fuse.
2. Description of the Prior Art
Current limiting fuses, such as that disclosed in U.S. Pat. No. 3,243,552, issued Mar. 29, 1966, to H. W. Mikulecky, conventionally comprise one or more fusible elements, each having a plurality of fusion points distributed along its length and embedded in an inert granular arc quenching material such as sand or finely divided quartz. Usually these fusible elements are in the form of thin silver ribbons having serially related portions of relatively small cross sectional area which determine the points where fusion of the fusible element is initiated on high magnitude fault currents. For example, the silver ribbons may be provided with a plurality of circular, spaced apart, perforations which determine these fusion points. When these fusible elements are subjected to high magnitude fault currents, all of the portions of small cross sectional area fuse or vaporize almost simultaneously, resulting in the formation of arclets in series which control the transient voltage across the fuse elements. The metal vapors at these fusion points rapidly expand to many times the volume originally occupied by the fusible element and are thrown into the spaces between the granules of inert filler material where they condense and are no longer available for current conduction. The current limiting effect results from the interaction of the metal vapors and the inert granular material surrounding the fusible element. The physical contact between the hot arc and the relatively cool granules causes the rapid transfer of heat from the arc to the granules, thereby dissipating most of the arc energy with very little pressure build-up within the fuse enclosure. The vapors of silver have relatively low conductivity unless their temperature is particularly high, and the temperature of the silver vapors is rapidly reduced by the sand filler until the vapors will not support a flow of current. The quartz sand particles, which are in the immediate vicinity of the arc, fuse and become partial conductors at the high temperature of the arc and form a fulgurite, or semiconductor. The fulgurite resulting from fusion and sintering of the quartz sand particles is in the nature of a glass body, and as it cools it loses its conductivity and becomes an insulator.
Generally, the fusible elements of high voltage current limiting fuses also include an "M" spot along a central portion of the fusible element to determine the time-current melting characteristics of the fuse when it is subjected to a low magnitude fault or overload current. This "M" spot is generally in the form of a bead of low melting temperature alloy such as tin-lead solder which is in intimate contact with the fusible silver ribbon. When a low magnitude overload current flows for a long period of time, the silver ribbon becomes hot enough to melt this alloy bead, and the amalgamation of the silver and the alloy bead produces a eutetic solid solution having a much lower melting temperature than that of pure silver, with high enough resistance to melt the ribbon at this point. This amalgamation process is very effective at long melting times, but at shorter times it is very slow responding. Consequently, at intermediate melting times, in the range of 0.1 to 1,000 seconds, these current limiting fuses do not coordinate very well with other types of overcurrent devices, such as cutouts and reclosers which have more inverse time-current characteristics in this range.